Practically all wild mammals, even healthy ones, carry at least a few parasites, internal or external or both. Most are also susceptible to at least some diseases caused by invasive microorganisms. The degree of inconvenience and debilitation accompanying infestation varies a great deal. Some parasites go unnoticed; others can cause intense irritation over a long period; a few are fatal. If parasites or diseases could affect weasel populations we should consider them here.
The best source of detailed information on this subject is a recent review commissioned by the New Zealand Department of Conservation (McDonald & Lariviere 2001), as part of its intensive program to develop a means of biological control of stoats (Chapter 13). In general, weasels are susceptible to various diseases, such as tularemia, canine distemper, Aleutian disease of mink, rabies, murine (but not ovine) sarcosporidiosis, and bacterial infections caused by Bartonella sp. and Borrelia burgdorferi. Another bacterial pathogen, Helicobacter mustelae, is widespread in both ferrets and stoats (Forester et al. 2003). Histological signs of some forms of disease in wild stoats in Britain have been described (McDonald et al. 2001), but practically nothing is known about the incidence or effects of any diseases and parasites on wild weasels of any species.
Bovine tuberculosis (TB) is a serious problem for dairy farmers, especially where there are continual "breakdowns" (reinfections of a cleared herd) from contact with infected wild mammals. Removal of the wild species harboring the disease becomes a high priority to farmers in such areas. In Britain the main targets are badgers, especially in southwest England; none of the 33 common weasels nor the 33 stoats examined between 1971 and 1985 was positive for TB (Anon. 1987). In New Zealand the main reservoir of infection is the huge population of introduced Australian brushtail possums. Stoats do pick it up from them, though much less often than do feral ferrets (in one survey only one of 62 stoats collected from a TB area was positive) (Ragg et al. 1995).
McDonald and Lariviere (2001) listed the helminth parasites recorded from stoats as Taenia mustelae, T. tenuicollis, Alaria mustelae, Molineus patens, M. mustelae, Trichinella spiralis, Troglotrema acutum, Capillaria putorii, Strongy-loides martis, Aleurostrongyluspridhami, Gnathostomus nipponicum, Mesocestoides lineatus, Filaroides martis, Dracunculus sp., and Acanthocephalis sp.
Skin parasites are common in most mammals. Lice, some ticks, and mites are hard to see, especially the larvae, and all we can say is that weasels do have some. The records are sparse and probably grossly inadequate, but for what they are worth we list them here. Common weasels have a specific louse, Trichodectes mustelae, and can carry the mites Demodexspp., Haemaphysalis longicornis, and Psorergates mustela (Tenquist & Charleston 1981). Stoats have a specific louse, Trichodectes ermineae, recorded in Canada, Ireland, and New Zealand (Jennings et al. 1982; Sleeman 1989a; King 2005b). They also carry the mites Eulaelaps stabularis and Hypoaspis nidicorva (both normally found on birds), Demodex erminae, Gymnolaelaps annectans, Haemophysalis longicornis, and Listrophorus mustelae. Some stoats show symptoms of mange caused by the mites Sarcoptes. In Ireland, a comprehensive examination of stoats detected the mite Neo-trombicula autumnalis (only the larva is parasitic, but one female carried 1,819 of them); the lice Polyplax spinulosa (of rats) and Mysidea picae (of corvid birds); and the ticks Ixodes hexagonus (a nest species; 266 larvae on one female), I. ricinus (common on rats), and I. canisuga (Sleeman 1989a). A new species of tick Ixodes gregsoni has been found on minks, martens, and weasels in Canada (Lindquist et al. 1999).
The only two parasites of weasels that have received considerable attention are the fleas, because they can tell us quite a lot about how weasels hunt and move about their home ranges, and Skrjabingylus nasicola, a nematode worm that causes dramatic lesions in the skull.
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